Suppose there is a user A which transfers some ethers to user B through a proper transaction.
So is it possible to alter the transaction. Something like going back to the block which contains the transaction and creating another chain with a new transaction in which ethers are not sent to User B. So, will the second transaction override the first one.
In short, I want to know what will happen to both of the transaction and how the blockchain will discard one and will keep the other?